The preparation of dimethyl ether from a H.sub.2 /CO mixture can be carried out either in two steps or in a single step. In the two-step preparation the H.sub.2 /CO mixture is converted in the first step into methanol by contacting it with a methanol synthesis catalyst, and subsequently the methanol formed is converted in a second step into dimethyl ether by contacting it with a dehydration catalyst. The development of the reaction in the two-step process may be rendered as follows. ##EQU1##
Among the methanol synthesis catalysts those containing copper, zinc and/or aluminum figure largely. They can be prepared by drying and calcining a co-precipitate obtained by adding a basic reacting substance to an aqueous solution in which the metals concerned are present in such quantities that the following requirements are met:
(a) the Cu/Zn atomic ratio is lower than 10, PA0 (b) the (Cr+Al)/Cu+Zn) atomic ratio is lower than 2, and PA0 (c) the Cu/(Cu+Zn+Cr+Al) atomic ratio is higher than 0.1. PA0 (a) the Cu/Zn atomic ratio is lower than 10, PA0 (b) the (Cr+Al)/(Cu+Zn) atomic ratio is lower than 2, and PA0 (c) the Cu/(Cu+Zn+Cr+Al) atomic ratio is higher than 0.1, PA0 (a) the Cu/Zn atomic ratio is lower than 10, PA0 (b) the (Cr+Al)/(Cu+Zn) atomic ratio is lower than 2, and PA0 (c) the Cu/(Cu+Zn+Cr+Al) atomic ratio is higher than 0.1. PA0 (a) an X-ray powder diffraction pattern in which the strongest lines are the four lines mentioned in Table A PA0 (b) in the formula which represents the composition of the silicate, expressed moles of the oxides, and which, in addition to SiO.sub.2, includes one or more oxides of a trivalent metal M chosen from the group formed by iron, aluminium, gallium and boron, the SiO.sub.2 /M.sub.2 O.sub.3 molar ratio is higher than 25.
It has been found that in the above-mentioned catalysts the activity for the conversion of a H.sub.2 /CO mixture into methanol is to a great extent dependent on the temperature at which the co-precipitation is carried out. According as the co-precipitation is carried out at a higher temperature, the activity of the catalysts obtained will be higher. It has further been found that for the preparation of catalysts having an acceptable activity for the conversion of a H.sub.2 /CO mixture into methanol on a technical scale the temperatures applied in the co-precipitation should be higher than 80.degree. C. Contrary to their activity, the stability of these catalysts for the conversion of a H.sub.2 /CO mixture into methanol is found to be virtually independent of the temperature applied in the co-precipitation. Both catalysts for which the co-precipitation was carried out at a high temperature and catalysts for which the co-precipitation was carried out at a low temperature have a high stability for the preparation of methanol from a H.sub.2 /CO mixture.
The single-step preparation of dimethyl ether from a H.sub.2 /CO mixture can be carried out by contacting the H.sub.2 /CO mixture with a mixture of a methanol synthesis catalyst and a dehydration catalyst. The development of the reaction in the single-step process may be rendered as follows: EQU 3H.sub.2 +3CO.fwdarw.CH.sub.3 OCH.sub.3 +CO.sub.2.
For the preparation of dimethyl ether from a H.sub.2 /CO mixture there is a marked preference for the single-step process rather than the two-step process, the reasons being the following. Firstly, the maximum achievable equilibrium conversion is considerably higher for the single-step process than for the two-step process. Further, as shown by the reaction equations given hereinbefore, the two-step process requires the use of a hydrogen-rich H.sub.2 /CO mixture, whereas for the single-step process a low-hydrogen H.sub.2 /CO mixture suffices as feed. This is of particular interest, since nature provides large amounts of material with a relatively low H/C ratio, such as coal, which when used as starting material for the preparation of H.sub.2 /CO mixtures, yield low-hydrogen mixtures. Finally, for application on a technical scale a single-step process is more attractive, obviously, than a two-step process.
An investigation into the single-step preparation of dimethyl ether over a mixture of a methanol synthesis catalyst and a dehydration catalyst, in which the methanol synthesis catalyst used was a catalyst for which the co-precipitation had been carried out at a temperature above 80.degree. C. has revealed that this catalyst mixture had a high activity, but a low stability. Further investigation showed that the stability of these catalyst mixtures is to a great extent dependent on the temperature at which the co-precipitation of the methanol synthesis catalyst present therein has been carried out. According as this co-precipitation has been carried out at a lower temperature, the catalyst mixtures have a higher stability. It was found that in order to compose catalyst mixtures having an acceptable stability for the conversion on a technical scale of a H.sub.2 /CO mixture into dimethyl ether, in the catalyst mixture use should be made of a methanol synthesis catalyst for which the co-precipitation has been carried out at a temperature below about 70.degree. C. Unlike their stability, the activity of these catalyst mixtures for the conversion of H.sub.2 /CO mixture into dimethyl ether is found to be virtually independent of the temperature applied in the co-precipitation. Both catalyst mixtures containing a methanol synthesis catalyst for which the co-precipitation has been carried out at a low temperature and those containing a methanol synthesis catalyst for which the co-precipitation has been carried out at a high temperature have a high activity for the preparation of dimethyl ether from a H.sub.2 /CO mixture.
The above-described investigation has led to the conclusion that there is a radical difference in the extent to which the temperature at which the co-precipitation of the methanol synthesis catalyst is carried out influences the performance of this catalyst, depending on whether this catalyst is used per se for the preparation of methanol, or whether it is used in the form of a catalyst mixture together with a dehydration catalyst for the preparation of dimethyl ether. It is now possible to compose catalyst mixtures for the single-step conversion of a H.sub.2 /CO mixture into dimethyl ether, which catalyst mixtures have both a high activity and a high stability.